Editing Test-and-set (section)

== Hardware implementation of test-and-set ==
[[DPRAM]] test-and-set instructions can work in many ways.  Here are two variations, both of which describe a DPRAM which provides exactly 2 ports, allowing 2 separate electronic components (such as 2 CPUs) access to every memory location on the DPRAM.

=== Variation 1 ===
When CPU 1 issues a test-and-set instruction, the DPRAM first makes an "internal note" of this by storing the address of the memory location in a special place.  If at this point, CPU 2 happens to issue a test-and-set instruction for the same memory location, the DPRAM first checks its "internal note", recognizes the situation, and issues a BUSY interrupt, which tells CPU 2 that it must wait and retry.  This is an implementation of a [[busy waiting]] or [[spinlock]] using the interrupt mechanism.  Since all this happens at hardware speeds, CPU 2's wait to get out of the spin-lock is very short.

Whether or not CPU 2 was trying to access the memory location, the DPRAM performs the test given by CPU 1.  If the test succeeds, the DPRAM sets the memory location to the value given by CPU 1.  Then the DPRAM wipes out its "internal note" that CPU 1 was writing there.  At this point, CPU 2 could issue a test-and-set, which would succeed.

=== Variation 2 ===
CPU 1 issues a test-and-set instruction to write to "memory location A".  The DPRAM does not immediately store the value in memory location A, but instead simultaneously moves the current value to a special register, while setting the contents of memory location A to a special "flag value".  If at this point, CPU 2 issues a test-and-set to memory location A, the DPRAM detects the special flag value, and as in Variation 1, issues a BUSY interrupt.

Whether or not CPU 2 was trying to access the memory location, the DPRAM now performs CPU 1's test.  If the test succeeds, the DPRAM sets memory location A to the value specified by CPU 1.  If the test fails, the DPRAM copies the value back from the special register to memory location A.  Either operation wipes out the special flag value.  If CPU 2 now issues a test-and-set, it will succeed.